An organic light emitting diode (“OLED”) display typically includes, in sequence: (1) a substrate made of glass; (2) a transparent anode made of indium tin oxide (“ITO”) and the ITO has an index of refraction (“n”) of approximately 1.8; (3) a hole transport layer (“HTL”) that has “n” of approximately 1.5; (4) an electron transport and light emissive layer (“emissive layer”) that has “n” of approximately 1.7; and (5) a cathode. When a forward bias voltage is applied, holes are injected from the anode into the HTL, and the electrons are injected from the cathode into the emissive layer. Both types of carriers are then transported towards the opposite electrode and allowed to recombine with each other in the display, the location of which is called the recombination zone.
Due to the refractive indices of the different layers, and the glass substrate, only a small percentage of the light emitted by the emissive layer is output from the display. One technique to increase the percentage of light output from the display is to use a resonant OLED structure, which is an OLED device that makes use of a microcavity. The mirrors needed to form the microcavity are provided by the metal cathode and a multi-layer stack of non-absorbing materials (e.g., a distributed Bragg reflector (“DBR”) stack). The resonant OLED display achieves greater percentage of light output and also greater light intensity thru constructive interference of wavelengths that are in resonance with the microcavity. The wavelength of the light output by the display is determined, in part, by the optical length of the microcavity, which can be manipulated by, for example, changing the thickness of the layers that make up the microcavity.
Unfortunately, microcavity devices have an emission spectrum that undesirably varies as a function of viewing angle from the display. That is, a blue shift in the emitted wavelength (i.e., a shift towards shorter wavelengths) occurs with an increase in the viewing angle from the normal to the emitting surface of the display. In microcavity devices, the distance between standing wave nodes of incident and reflected waves decrease with an increase in viewing angle. Thus, to match the characteristic dimension of the cavity requires shorter wavelengths. Accordingly, the peak emitted wavelength emitted by the microcavity may decrease by about 20 to 45 nm with a 40° shift in viewing angle from the normal to the emitting surface of the display (i.e., the normal to the emitting surface of the display means that the emitted light is viewed at 0° viewing angle). This blue shift limits the use of the resonant OLED structure in a number of important applications, such as displays and traffic lights, where visual perception and impressions are important.
FIG. 1 shows the dependence of color on the viewing angle when a microcavity is used in the OLED display. From a viewing angle that is normal to the emitting surface of the display (i.e., viewing angle of 0°), the peak emitted wavelength is at 540 nm. However, at 20° viewing angle, the peak emitted wavelength has blue shifted by about 15 nm. At 40° viewing angle, the blue shift is worse—the peak emitted wavelength has blue shifted by about 25 nm from the peak emitted wavelength at the 0° viewing angle. The blue-shifting results in a perceived color change of the light output by the microcavity OLED display and this color change is unacceptable.
Because of the advantages of using a microcavity such as increased light intensity, increased percentage of light output, and improved color purity, it is desirable to have an OLED device that uses a microcavity but it should be designed to minimize or eliminate the color change due to a change in the viewing angle.